已知x^2+y^2=7,xy=-1,求代数式：5x^2-(3xy+4y^2)-(11xy-2y^2)-7x^2

已知1/x-1/y=5,求分式y+xy-x/2x+7xy-2y的值. 已知1/x-1/y=5 求分式-x+xy+y/2x+7xy-2y 已知1/X-1/y=5,求分式(y+xy-x)/(2x+7xy-2y)的值 已知X+y=6,xy=7 求x^2y+xy^2=已知X+y=6,xy=7 x^2y+xy^2 已知3/(x-y)=1/xy 求(-x-2xy+y)/ (2x+3xy-2y) 已知xy-x-y=11,求(xy-1)^2+(x+y-2)(x+y-2xy)的值. 已知1/x+1/y=3,求分式(3x-3xy+3y)/(x+xy+y)的值我知道第一步是两边都乘以xy得x+y=3xy,然后怎么代进去,求回答! 已知1/x-1/y=2,求3x+7xy-3y/2x-3xy-2y答案是1/x-1/y=2(y-x)/xy=2 得：x-y=-2xy(3x+7xy-3y)/(2x-3xy-2y)=[3(x-y)+7xy]/[2(x-y)-3xy]=(-6xy+7xy)/(-4xy-3xy)=xy/(-7xy)=-1/7 谁能帮我解释一下=[3(x-y)+7xy]/[2(x-y)-3xy]=(-6xy+7xy)/(-4xy-3xy) 是怎 已知,x-y=2,xy=7 求x^2 y-xy^2 已知xy^2=-2求xy（2x^3y^7+5x^2y 已知x+y=0,xy=-7,求x^2y+xy^2 已知x+y=2,xy=7求x²y+xy²的值 已知整数x和y,满足x^+xy-2y^=7,求xy的值 已知x+y=-1,xy=-2,求代数式-5（x+y)+(x-y)+x(xy+y)的值 .已知:x-y=1,xy=-2.求:(-2xy+2 x+3y)-(3xy+2y-2x)-(x+4y+xy)的值 已知x y=1,xy=-3,求2xy-4x 4(y-2(xy-x)) 已知x-xy=8,xy-y=-9,求x+y-2xy的值 已知x=3y,试求（x-y)^2+2xy-y^2/x^2+7xy+y^2